1. Field of the Invention
The present invention relates to semiconductor wafer processing equipment and, more particularly, to a diagnostic pedestal assembly for measuring ion current uniformity and DC bias uniformity within a reaction chamber of such equipment.
2. Description of the Background Art
Ion flux and wafer bias voltage are important parameters of a plasma contained by a reaction chamber within a semiconductor wafer processing system. These parameters indicate the effectiveness of the semiconductor wafer processing system to process a wafer uniformly across the wafer. Specifically, the ion flux and wafer bias voltage affect the uniformity of an etch process (as well as deposition uniformity in chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes) and indicate the potential for damage to a wafer due to a non-uniform plasma. Since these parameters are so important to the etch and/or deposition process, the measurement of both ion current and wafer bias voltage at a given location within the chamber is important to characterizing the effectiveness of the plasma in processing the wafer.
Typically, to measure ion current, an ion current probe, similar to a Langmuir probe, is used. To measure the distribution of current at the surface of a wafer, one or more current probes are affixed to one surface of a placebo wafer, i.e., a silicon disk having the same size and shape as a semiconductor wafer. The placebo wafer is then positioned within a semiconductor processing system in a similar location as a semiconductor wafer would typically be located. Once a plasma is generated by the processing system, the ion current probes are biased negatively to collect ions from the plasma. Consequently, an electric current is produced in a wire attaching the probe to a current meter. The measured current is indicative of the number of ions incident upon the current probe at that location on the placebo wafer. By judiciously positioning the current probes in an array on the surface of the placebo wafer, the ion currents measured at each individual current probe are combined to estimate the ion current distribution over the surface of the placebo wafer. This current distribution is indicative of the ion flux within the plasma.
To measure DC bias voltage, the conductive probes are unbiased, RF filtered and are each coupled to a volt meter. The measured voltage is indicative of the DC bias voltage on the surface of the wafer at the particular locations of the probes. Judicious placement of the voltage probes in an array on the surface of the placebo wafer provides an estimate of the DC bias voltage distribution over the surface of the placebo wafer.
Although the use of a placebo wafer carrying a plurality of diagnostic probes is very useful in monitoring and measuring a plasma within a reaction chamber, experience has shown that a placebo wafer having surface mounted current and voltage probes is not a useful diagnostic tool to characterize a plasma within a high-cathode-powered reaction chamber i.e., a chamber having a substantial amount of power coupled to the plasma from the cathode. One problem associated with placebo wafer utilization in a high-cathode-powered chamber is the significant degree of arcing that occurs due to voltage differentials created on various components of the diagnostic wafer. Such arcing is detrimental to the diagnostic wafer, the measurement equipment and the semiconductor wafer processing equipment itself.
Therefore, a need exists in the art for apparatus that measures ion flux uniformity and DC bias uniformity in a high-cathode-powered reaction chamber of a semiconductor wafer processing system.